1. Field of the Invention
This invention generally relates to a method and apparatus for producing via holes in polymer dielectrics without use of a mask. More particularly, the invention relates to a method for packaging electronic integrated circuit chips disposed on a substrate wherein via openings are produced in a polymer film overlaid on the chips to facilitate electrically interconnecting the chips through the vias thus formed.
2. Description of the Prior Art
Formation of via holes to allow for electrical connection between two or more layers of conductor separated by a layer of dielectric material has heretofore been accomplished by several methods, each of which has limitations.
In one prior method, via holes are formed by depositing a metal masking layer over the dielectric layer, patterning the masking layer to expose areas of the dielectric layer where via holes are desired, and then selectively etching the exposed areas of the dielectric layer through the masking layer. Either chemical or plasma etching techniques are used to remove the dielectric from the exposed areas, although use of either technique has its own unique drawbacks. Certain dielectric materials cannot conveniently be chemically etched since they absorb chemical etchants, causing damage to the dielectric material underlying the metal masking layer. In addition, some chemical etchants are known to attack metallization, thus requiring special attention when the via being etched is designed to expose an aluminum interconnect pad on a very large scale integrated circuit (VLSI) chip. The anisotropic nature of plasma etching tends to produce barrel-shaped via holes. Depositing metallization in barrel-shaped via holes is very difficult. Moreover, thickness variations in the dielectric layer being etched may result in excessive plasma etching in certain areas with marginal etching in others.
In another known method, photo-patternable polymer materials are applied to a substrate and then exposed to light in the areas where via holes are desired. After exposure, the polymer is developed, and baked at high temperatures. This method has three prominent limitations. First, it may only be used with photo-patternable materials. Second, the thickness of material which can be photopatterned is limited to approximately a maximum of 5 microns, due to the inherent properties of the material and sensitizers therefor. Third, the photo-patternable materials must be deposited in liquid form and then merely dried such that they may be easily developed. Prereacted film overlays, such as Kapton.RTM. polyimide film available from E.I. du Pont de Nemours and Company, Wilmington, Del., cannot be used.
Another prior method involves forming pillars of conductor material on a substrate and filling in around the pillars with a dielectric. In this method, a layer of metallization on a substrate is first patterned and the metal is then built up by electroplating to form the pillars. Polymer material is then sprayed or spun onto the substrate in multiple coats, leaving sufficient curing time between coats to allow solvent and by-products of the curing process to escape. Enough coats are applied to completely cover the conductors on the substrate, but to barely cover the pillars. Following a short etch sufficient to uncover the top surfaces of the pillars, the pillars can function as metal-filled vias. This method has the disadvantages of requiring an excessive number of steps and the use of more difficult wet processing techniques.
Eichelberger et al. U.S. Pat. No. 4,714,516, issued Dec. 22, 1987 and assigned to the instant assignee, discloses the use of an argon ion laser to form vias. Selected areas of the dielectric are damaged with laser energy and the damaged areas are then etched with oxygen plasma. The damaged portion of the dielectric etches at a much higher rate than the undamaged portion. One qualification on use of this approach arises because hole depth is limited by the amount of damage at the surface; that is, the via hole must be at least as wide at the top as the thickness of the dielectric material to be drilled. As a practical matter, via hole depths are limited to about 10 microns; therefore, thicker dielectric material requires extra steps in the formation of a via. For example, via formation would require at least the steps of damaging the dielectric with the laser, cleaning out the resulting hole with plasma, and thereafter damaging the dielectric at the bottom of the hole with the laser, followed by again cleaning out the hole with plasma. Another limitation to this approach is that the surface of the dielectric layer is exposed to excessive amounts of plasma. A substantial amount of plasma is required to clean out the holes and the undamaged dielectric surface tends to become rough from exposure to the plasma. The rough surface makes subsequent metallizations difficult to pattern.
Pulsed lasers such as excimer and doubled YAG lasers have been used to directly ablate dielectric layers. By this technique, laser energy is absorbed in a thin layer of the dielectric in sufficient amounts to evaporate or ablate the thin layer of dielectric where the laser beam impinges. Multiple pulses are required to ablate holes of increased depth. Care must be taken to avoid ablating the underlying metal with laser pulses since the energy necessary to heat the polymer to ablation temperatures can also destroy the metallization underlying the dielectric. To effectively practice this method, only very small thicknesses of dielectric may be removed per pulse and, as a result, the method requires an excessive number of pulses. Lasers emitting visible light are not acceptable for this ablation technique because the penetration depth is too great.
The direct laser ablation technique limits electronic package manufacturers to use of circular via holes. Round vias, however, are not as good as rectangular vias since they limit the area available for electrical contact; that is, rectangular vias are easily enlarged into larger rectangular vias without leaving residual materials in the bottom of the holes. Yet.the shape of the via holes formed using this technique is unavoidably circular because the laser energy is a focussed version of the laser cavity. Even if a non-circular mask opening is used as an aperture, the laser lens will nevertheless focus the energy as a circle. Changing via hole sizes requires a sophisticated zoom lens which can change focal length and f number to balance the conflicting optical requirements for hole size variation and constant depth of focus.
Direct laser ablation techniques also do not allow for creation of sufficiently narrow via hole sizes in thick dielectrics, i.e., dielectrics too thick to be patterned by conventional photoresist techniques. The top of the hole is necessarily larger for larger thicknesses of dielectric, with the result that hole size at the top becomes too large when working with thick dielectrics. Yet thick dielectrics are desirable for many applications. When working with self-standing dielectric films, thicker films are easier to handle. Thick dielectrics are advantageous in systems where capacitance of conductor lines must be minimized. In addition, thick dielectrics are preferred for compliance with radiation hardness requirements - the dielectric must be sufficiently thick to absorb the secondary electrons generated by conductors overlying semiconductors.